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It should be controlled (somewhat) by the ASTM standards for the geometry but in practice I have found that they aren't. Guerrilla air ones used to be way longer than anyone else and cause us a lot of issues with ASA's and on/offs.

The force is going to depend on the surface areas and the tank output pressure. I have never measured them to be honest but I bet that varies quite a lot too. You may need to measure the specific tank you want to work with.

If the pin sealing surface is a .25" od (far larger then in reality,) the area is .05in^2, so at 1000psi that is 50lbs.

I want to say sealing surface is less then that, but I haven't taken a tank apart in quite a while. My guess is 20-40lbs, but distance for a tank pin depresser on the ASA is .075" or so, and travel of the pin only needs to be about .05" of travel (or should be, if the tanks are made to ASTM standards) so figure running a .5" arm (1:10 ratio) you can reduce the arm pressure to 2-4lbs.

I'll have to find my 2005 copy of the standard, but the 1996 version does not have any requirements as far as the force on the valve core is concerned. It can't extend past the face of the male fitting, has to reach full open with .070" of depression, and has to accept a depression of at least .100" without taking damage.

I have reversed drawings of the ASTM standard shared on my Google Drive:

Well, I was just thowing stuff out there to show the extreme example, and how it related to the rest of the action. It is dependant on the pin sealing surface diameter and pressure of the tank.

In the case of the Ninja, the pin is smaller for the contact/sealing area, and the pressure is lower. So it should be quite a bit lighter then that. With a 1/8" sealing surface you have .01"in^2 and at 500lbs that is 5lbs of pressure.